Admixtures

Admixtures are chemicals added in small doses, usually under a few percent of cement weight, to change the properties of concrete. They are now part of almost every structural mix. Used well, they make concrete stronger, more durable and easier to place; used carelessly, they can cause slow setting, segregation or loss of strength.

Water-reducing admixtures (plasticisers) disperse cement particles so the same workability is achieved with less water, typically around 5 to 12 percent less. Superplasticisers, based on sulphonated naphthalene formaldehyde or polycarboxylate ethers, cut water by 20 percent or more and make flowing and self-compacting concrete possible. Retarders delay setting for hot weather and long pours; accelerators speed early strength for cold weather or precast work. Air-entraining agents create tiny stable bubbles that protect against freeze-thaw. Specialist products include corrosion inhibitors, shrinkage-reducing agents, viscosity modifiers and waterproofing (crystalline) admixtures.

Mineral admixtures, or supplementary cementitious materials, are a different family: fly ash, ground granulated blast furnace slag, silica fume and metakaolin. They react with lime released by cement hydration to form extra binding gel, reducing permeability and heat while improving long-term strength and sustainability. Compatibility between a chemical admixture and a given cement must be checked by trial, using tests like the Marsh cone to find the saturation dosage.

On site, admixtures are dispensed by calibrated pumps at the batching plant, not poured by hand into the truck. Slump retention over the haul time should be checked, because some admixtures lose effect quickly in hot weather and the crew may then be tempted to add water.